This invention relates to high frequency antenna systems and more specifically to wideband feeds for use in such antenna systems.
U.S. Pat. No. 4,042,935, entitled "Wideband Multiplexing Antenna Feed Employing Cavity Backed Wing Dipoles," by J. S. Ajioka and G. I. Tsuda, and assigned to a common assignee with this application, describes a nested cup dipole feed for a circularly polarized antenna. The feed covers multiple octave bands. Between each octave, or at the crossover points in frequency, the gain or sensitivity drops by about 7 dB. As represented in FIG. 2 of this patent, the outer four printed circuit elements cover an octave band. A diagonal pair is fed by a balun to provide linear polarization. The orthogonal pair is also fed by a balun to provide orthogonal linear polarization. For the circular polarized application, the two orthogonal linearly polarized dipoles are fed by a 90 degree hybrid. Another set of four elements placed 45 degrees with respect to the first set covers the second octave band. The third set of four is again placed 45 degrees with respect to the second set but is colinear with the first set. The elements for each band are positioned 45 degrees from their respective adjacent bands. When the feed is used with a parabolodial reflector with a focal distance to diameter ratio of between 0.3 to 0.45, the average efficiency ranges from 40% to 50%. At the band or frequency crossover, the efficiency drops to about 10%.
The applicability of nested cup dipole feed of U.S. Pat. No. 4,042,935 could be increased if the polarization can be made collinear. For instance, the feed of U.S. Pat. No. 4,042,935 cannot be used for an offset reflector because the dipoles for all bands cannot be aligned radially or circumferentially for all bands. If the dipoles (polarization) are not aligned properly, the asymmetry created by the offset reflector causes depolarization which results in coupling between both dipoles. This causes the efficiency to degrade and the beam to squint as a function of frequency and polarization. Another advantage of collinear arrangement is that there are many cases where vertical and horizontal polarization (in space) are required rather than slant 45 degrees. Other applications may require collinear dipoles with staggered crossover tuning. By tuning one dipole differently with respect to the orthogonal ones, a large efficiency decrease can be avoided for at least one linear polarization at the crossover frequencies. In other words, frequency staggering can be accomplished.
There are many applications requiring that the polarization from one band to another be aligned; that is, all vertical and all horizontal.
It is therefore an object of the present invention to provide a nested cup dipole feed which provides collinear polarization for all bands.
A further object is to provide a nested cup dipole feed which enables frequency staggering of one linear polarization with respect to the orthogonal linear polarization if required, thus permitting at least high gain for one polarization.